A correlated series of experimental and theoretical studies regarding hormone receptor interactions has been undertaken. A number of model systems have been utilized, including the receptors for enkephalin, opiates, benzodiazepines, LHRH, and Insulin. By development of appropriate mathematical models, computer program for curve fitting and data analysis, and of improved experimental designs, we have been able to characterize several of these ligand-receptor systems. Special attention has been given to the cases of 1) multiple ligands reacting simultaneously with multiple classes of sites, 2) the presence of multiple sub-sites on receptor areas; 3) the interaction of ligand-binding with an ionophore; 4) the interaction of receptors for different ligands on the same cell membrane.